The present invention relates generally to a method and system for geophysical exploration and more particularly to a method and system for obtaining selected physical properties from samples of the earth's subterranean formations surrounding a wellbore and for obtaining inverse logs of the selected physical properties along the length of the wellbore for evaluating the oil and gas bearing potential of the earth's subterranean formations.
In the continuing search for oil and gas, explorationists have developed a wide array of geophysical exploration methods for imaging the earth's subterranean formation structures and displaying those images as well as providing estimates of selected formation characteristics such as lithology, porosity, density, and velocity of propagation of seismic energy in the earth's formations. Exemplary of such geophysical exploration methods are seismic surveys, gravity surveys, and magnetic surveys. The principal geophysical method for oil and gas exploration within the earth's subterranean formation is seismic exploration. Simplistically, seismic exploration comprises imparting seismic energy into the earth and recording the earth's response thereto. The earth response can include both refracted and reflected seismic energy. As the seismic energy propagates through the earth, it is controlled by the physical properties of the rocks and the fluids contained therein. Unfortunately, explorationists cannot directly measure these physical properties which constrain the propagation of seismic energy from the recorded seismic survey data. Rather, seismic exploration techniques merely provide a measure of the time it takes the seismic energy to traverse some path. In fact, it is often times impossible to precisely ascertain the path taken by the seismic energy. In view of such uncertainties, how is it that explorationists are able to estimate formation lithology, fluid content or even simple, reconstruct formation geometries through which the seismic energy propagates with only a measure of time? Quite simply, the answer is explorationists cannot without the aid of additional information such as estimates of the velocities of propagation of seismic energy in rock and rock density as well as other rock and fluid physical properties. Methods for estimating the velocity of propagation and a density as a function of depth are generally known to those skilled in the art; however, velocity estimations are the weakest element in the inversion of seismic time data.
Often times, wells which have been drilled based on geophysical survey data are logged using one or more of the well logging techniques such as acoustic logging, resistivity logging, neutron logging, etc. Results of such well logging techniques can provide the explorationist with selected physical properties generally over regions of interest along the wellbore length. With such well logging techniques, explorationists are able to obtain better estimates of the formation physical properties for evaluating geophysical survey data. However, it must be recognized that such well logging techniques still only provide an indirect measure of the formation physical properties of interest and are generally limited to only selected regions of interest within a wellbore.
Occasionally, core samples are obtained from limited regions of interest within a wellbore for transport to a central laboratory so that physical properties of the formations adjacent the borehole, such as fluid content of the sample, (U.S. Pat. No. 2,458,093), oil content of the sample (U.S. Pat. No. 2,531,083), and electrical resistivity of the sample (U.S. Pat. No. 2,613,250) can be obtained. Unfortunately, such core samples generally represent only a small fraction of the formational lithologies traversed by the wellbore along its length. Moreover, the delay associated with transporting the samples to the central laboratory can preclude the measurement of properties of certain friable lithologies or result in seriously degraded measurements of certain properties. During the drilling process, chips which have broken away from the subsurface formations can be captured from the drilling fluid at the surface for analysis. Typically, explorationists have identified the lithologies of the subsurface formations from such chips. However, determining the exact depth in the wellbore from which the chips originated is generally not possible due to the very nature of their transport to the surface. Consequently, core samples and chip samples fail to provide either a regular sampling of borehole formation lithologies along the length of the wellbore or fail to accurately determine the depth of origin of such samples so that logs of formation characteristics, as a function of depth over generally the entire wellbore length, cannot be produced. Moreover, the very nature of the chip samples can often times preclude measuring certain formation characteristics (for example, measurement of rock velocity or compressive strength).
In spite of current geophysical exploration methods, explorationists are not provided with direct measures of the physical properties of the formations surrounding a wellbore along its length. Rather, attempts to evaluate and interpret geophysical survey data must generally depend on estimates of formation physical properties or on indirect measures of the formation physical properties from only limited regions of interest in the earth's subsurface. Consequently, explorationists are unable to accurately calibrate the vertical extent of their geophysical survey data with direct measures of the earth's physical properties so as to better evaluate the oil and gas bearing potential of the earth's subterranean formations.
In order to overcome the difficulties noted above, the present invention provides a novel method and system of obtaining selected physical properties from samples of the earth's subterranean formations surrounding a wellbore and developing inverse logs of the selected physical properties along the length of the wellbore. Explorationists can employ such inverse logs of the selected physical properties to perform basin analysis whereby the physical properties obtained at one or more wellbores can be used to calibrate geophysical survey data at each wellbore location and then extrapolate such physical properties to other locations in the basin with the geophysical survey data so as to evaluate the oil and gas bearing potential of the earth's subterranean formations.